Power balanced optical add/drop multiplexer and power balancing methods therefore

ABSTRACT

To improve a power balance of an optical add/drop multiplexer, an add amplifier is optically coupled to an add path of an optical add/drop module. A through loss associated with a signal passing through the add/drop module, a drop loss associated with a signal travelling a drop path of the add/drop module, and an add loss associated with a signal travelling an add path of the add/drop module are known or otherwise calibrated values. A dropped channel power measurement is used in conjunction with the through loss, add loss, drop loss, number of dropped channels and number of added channels to determine an add path amplification value. The gain of the add amplifier is controlled according to add path amplification value so that the power level of the added channel(s) substantially matches the power level of the WDM signal output from the add/drop multiplexer. Furthermore, the gain profile of the add amplifier preferably matches a gain profile of a signal input to the add/drop module. In this way, the power level and gain profile of the added channel(s) can be controlled to match the power level and gain profile of the signal passing through the add/drop module.

BACKGROUND OF THE INVENTION

The present invention relates to an optical add/drop multiplexer (OADM)and a control method therefore. More specifically, the invention relatesto an optical add/drop multiplexer with a controlled add amplifier and amethod of controlling an add amplifier of an OADM. The invention alsorelates to an optical add multiplexer (OAM) and a control methodtherefore.

Wavelength division multiplexing (WDM) is a burgeoning field. The reasonso much interest surrounds WDM is the ability to transmit a large amountof information. Each of the wavelengths in a WDM system may carry inexcess of 40 Gb/sec and advances in the WDM field are expected toincrease the capacity of each wavelength dramatically. Moreover, thenumber of channels or operating wavelengths is also expected to increasedramatically.

One of the common problems faced by WDM systems and the vendors makingWDM equipment is spectral flatness. A spectrally flat signal in whichall of the channels have substantially the same gain is preferredbecause such a signal may be transmitted a longer distance withoutregeneration while still permitting the constituent channels to bedistinguished from one another by a receiver.

To that end, WDM equipment vendors have invested large sums inventinggain-flattened optical amplifiers designed to impart substantially equalgain to signals over the operating wavelength range.

As recognized by the inventors, however, problems occur when an OADM(optical add drop multiplexer) is used to add and drop channels.Specifically, the power balance may be significantly affected whenchannels are added or dropped and added to a WDM signal.

One method of achieving the desired power balance for a WDM system withan OADM is disclosed in U.S. Pat. No. 6,069,719 (the Mizrahi patent). Inthat patent, a optical amplifier is placed before or after the OADM tobalance or adjust the power levels of the WDM signal.

SUMMARY OF THE INVENTION

Although the Mizrahi patent does help flatten or otherwise equalize thepower levels of the WDM channels, the technique disclosed therein is notsufficient in all cases.

One such case is when the power level of the channels to be added isquite different than the power levels of the main WDM signal. In such acase, placing an amplifier at the output of the OADM may not provide asufficiently flat power balance.

Another case occurs due to the add loss, drop loss and through loss ofthe OADM which may have significant values. “Drop Loss” as it is definedherein is the power loss incurred by a signal passing through an OADMmodule drop path. Likewise, “Add Loss” as it is defined herein is thepower loss incurred by a signal passing through an OADM module add pathand “Through Loss” is defined herein is the power loss incurred by asignal passing through an OADM module through path.

Another case occurs when the numbers of channels being added, droppedand passed vary. In such a case, the power balance of the WDM signaloutput from the OADM may be negatively impacted.

The present invention solves the above-identified and other problemswith conventional OADMs and OAMs by providing power balanced systems andmethods.

Therefore, the inventors hereby disclose an optical communicationsapparatus for power balancing a wavelength division multiplexed (WDM)signal output from an add/drop module adding at least one channel to anddropping at least one channel from a signal input thereto, including: again element optically coupled to the add/drop module and to an addchannel port receiving the at least one channel to be added; the gainelement imparting optical gain to the at least one channel received fromthe add channel port; a controller operatively coupled to the gainelement, the controller receiving a dropped channel power measurement ofthe at least one dropped channel dropped from the add/drop module; thecontroller determining an add path amplification value based on thedropped channel power measurement, a through loss associated with asignal passing through the add/drop module, a drop loss associated witha signal travelling a drop path of the add/drop module; and an add lossassociated with a signal travelling an add path of the add/drop module;and the controller controlling the gain element according to the addpath amplification value.

The controller may also receive a number of channels to be added by theadd/drop module, a number of channels to be dropped by the add/dropmodule, or both. This channel count information may then be used to helpdetermine the add path amplification value.

In order to measure the dropped channel power, the invention may use acoupler optically coupled to a drop output of the add/drop module, anoptical-to-electrical converter optically coupled to the coupler, theoptical to-electrical coupler receiving a portion of light from the atleast one dropped channel. The controller may then determine the droppedchannel power measurement from an output of the optical-to-electricalconverter.

To more accurately control the add path amplification, the controllermay also receive an added channel power measurement of the least oneadded channel being added to the input signal by the add/drop module.The controller may then feedback control the gain element based on theadded channel power measurement and the add path amplification value.

In addition to power matching, the invention may also use a gain elementhaving a gain profile substantially matching a gain profile of a signalinput to the add/drop module.

Alternatively, an input amplifier may be optically coupled an input portof the add/drop module and receive a plurality of input channels. Inthis case, the gain element may be designed to have a gain profilesubstantially matching a gain profile of the input amplifier.

A drop amplifier may also be optically coupled to the drop port of theadd/drop module to amplify the dropped channels.

In addition, an output amplifier may be optically coupled to theadd/drop module to amplify the output of the add/drop module. Thisoutput amplifier may also perform gain flattening amplification for thesignal output from the add/drop module.

There are a variety of ways to construct the gain element. One such wayis to use an add amplifier. The controller may then control the addamplifier according to the add path amplification value.

Alternatively, the gain element may include an add amplifier and avariable optical attenuator. The controller may then control thevariable optical attenuator and/or the add amplifier according to theadd path amplification value.

The invention also includes a method of power balancing a wavelengthdivision multiplexed (WDM) signal output from an add/drop module addingat least one channel to and dropping at least one channel from a signalinput thereto, including: receiving a dropped channel power measurementof the least one dropped channel dropped from the add/drop module;determining an add path amplification value based on the dropped channelpower measurement, a through loss associated with a signal passingthrough the add/drop module, a drop loss associated with a signaltravelling a drop path of the add/drop module; and an add lossassociated with a signal travelling an add path of the add/drop module;and controlling an add path amplification of the add path according tothe add path amplification value.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a block diagram of an optical add drop multiplexing systemaccording to the invention;

FIG. 2 is a block diagram of another optical add drop multiplexingsystem according to the invention;

FIG. 3 is a block diagram of yet another optical add drop multiplexingsystem according to the invention;

FIG. 4 is a block diagram of a still another optical add dropmultiplexing system according to the invention;

FIG. 5 is a block diagram of an optical add multiplexing systemaccording to the invention;

FIG. 6 is a high level flowchart illustrating an optical add dropmultiplexer control methodology according to the invention:

FIG. 7 is a graph illustrating a typical optical amplifier gain profilethat is useful for illustrating the concepts of the invention;

FIG. 8 is a graph illustrating a gain profile a signal passing throughan optical add drop multiplexer after dropping channels;

FIG. 9 is a graph illustrating a gain profile of channels to be added toa signal before amplification by an add amplifier according to theinvention;

FIG. 10 is a graph illustrating a gain profile of channels to be addedto a signal after amplification by an add amplifier according to theinvention; and

FIG. 11 is a graph illustrating a gain profile of channels to be addedto a signal after amplification by an alternative add amplifieraccording to the invention.

DETAILED DESCRIPTION OF INVENTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. Also, the following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents thereof.

The expression “optically communicates” as used herein refers to anyconnection, coupling, link or the like by which optical signals carriedby one optical system element are imparted to the “communicating”element. Such “optically communicating” devices are not necessarilydirectly connected to one another and may be separated by intermediateoptical components or devices.

Furthermore, the term “connection” and “operative connection” as usedherein is a relative term and does not require a direct physicalconnection. For example, controller 70 may communicate with otherelements by using a communication pathway over a fiber span, an Internetconnection, or wireless communication pathway. Such alternativecommunication pathways may be convenient in view of the potentiallyphysically distinct locations that may be provided for the variouselements.

FIG. 1 illustrates an OADM system 10 according to a first aspect of theinvention. An exemplary construction of the invention, as shown in FIG.1, includes an add/drop multiplexing (ADM) module 30. The ADM module 30may be optically coupled mid-span such as between an input amplifier 20and output amplifier 60 shown in FIG. 1. It is to be understood that theinput and output amplifiers 20, 60 are not necessary for the inventionto operate and that the location of the ADM module 30 shown in thefigures is exemplary in nature.

The ADM module 30 is, by itself, a conventional element that may beconstructed in a variety of fashions including a variety of components.For example, the ADM module may be constructed with a static add/dropmultiplexer as described in U.S. Pat. No. 6,069,719; with the add/dropmultiplexers shown and described in U.S. Pat. Nos. 6,002,503; 5,778,118;5,748,349; or 5,982,518; or with any other known or future-developedOADM. At a minimum, the ADM module 30 should have the capability to addone or more channels to an available or otherwise unoccupied channelslot(s) of a WDM signal. The ADM module 30 may also have the capabilityto drop one or more channels to make a channel slot available to thesignal to be added.

The ADM module 30 will typically utilize wavelength selective elementssuch as Bragg gratings, thin film filters, and/or arrayed waveguidegratings to add and drop channels. While the particular construction ofthe ADM module 30 is largely irrelevant to the invention, a materialpoint is that the ADM module will have various optical losses associatedwith the three main optical signals paths (add, drop and through).

More specifically and as indicated on FIG. 1, the ADM module 30 has anassociated Drop Loss which is the power loss associated with a signalpassing through the drop path of the ADM module 30. The ADM module 30also has an associated “Through Loss” which is the power loss incurredby a signal passing through the ADM module 30. The ADM module 30 alsohas an Add Loss associated therewith that is the power loss associatedwith a signal passing through the add path of the ADM module 30.

The drop, through, and add paths may only include the ADM module 30 asshown in FIG. 1 and the respective internal drop, through and add pathswithin ADM module 30. The add, drop and through paths may also includeother components such as connectors, isolators, and fiber situated alongthe respective paths and may be either internal or external to the ADMmodule 30.

The add, drop and through losses are preferably measured before thesystem is installed. In other words, the add, drop and through lossesare known, calibrated values stored within or retrievable by thecontroller 70.

As further shown in FIG. 1, the OADM system 10 also includes a dropamplifier 40 optically coupled to a drop output port of ADM module 30and an add amplifier 50 optically coupled to an add input port of theADM module 30.

The drop amplifier 40 and add amplifier 50 may be constructed with avariety of known or future developed amplifiers such as Erbium dopedfiber amplifiers, other rare-earth doped fiber amplifiers, Ramanamplifiers, Brillouin amplifiers or a combination thereof having one ormore stages of amplification. As is also known in the art, one or morepump lasers are typically coupled to the active optical fiber (e.g.rare-earth doped fiber) in one or more stages of each amplifier toexcite the fluorescent material or to stimulate Raman and/or Brillouinamplification. In general, the pump light is of sufficient magnitude andthe composition of optical fiber being pumped is such that the opticalsignals output from each stage are amplified.

Preferably, the add amplifier 50 has a gain profile similar to the gainprofile of input amplifier 20 or otherwise similar to the gain profileof a signal input to the ADM module 30. The reason for this is discussedbelow in more detail. Briefly, it is preferable to substantially matchthe gain profiles because the added channels should be amplified tomatch the WDM signal output from the ADM module 30 according to theconcepts of the present invention.

Various power measurements are taken by the invention (as represented bycouplers 22, 42, and 52) and supplied to controller 70. Couplers 22, 42,and 52 may constructed with a conventional tap or splitter so that asmall amount of light (e.g. approximately 2% of the input power fedthereto) is fed to the an optical-to-electrical (O/E) converter 72 thatmay be constructed with a conventional element such as a photodetector.The electrical signal may then be converted to a digital signal byanalog-to-digital (A/D) converter 74.

In practice, there may be one O/E converter 72 and one A/D converter 74per power measurement point. Moreover, the A/ID converter 74 would beunnecessary if the controller 70 is implemented with analog circuitry.

Controller 70 may be constructed in a variety of fashions utilizingelectronic components, optical components, computer hardware, firmware,software or a combination thereof. Controller 70 may also utilizeprocessing resources of an existing controller such as a network elementcontroller, terminal controller, or network managing controller thatalready exists in the communication system to which the system 10, 11,12, 13 or 14 is connected.

It is to be understood that a variety of devices and methods can beutilized to measure power at certain points in the optical path and thatthe couplers (22, 42, and 52) shown in the figures are but one exampleof these alternatives. Indeed, the amps (e.g. 20, 40, 50 and 60) as wellas the ADM module 30 may have the capability to measure input and outputpower levels and supply the measurement or a value indicative thereof tocontroller 70.

A relevant point is that controller 70 is supplied with various powerlevel measurement values. Specifically, the controller 70 receives powerlevel measurement values via couplers 22 and 42 indicative of the inputpower level and dropped channel power level.

In the preferred embodiment, the controller 70 also receives ameasurement of the actual power output from the add amp 50 via coupler52. While coupler 52 is shown in dotted lines to indicate that thiselement is optional, including coupler 52 or its equivalent to measurethe power output by add amp 50 permits the controller 70 to moreaccurately control the add amplifier 50 through a feedback loop.

The controller 70 also stores or receives the following values: Droploss, Add Loss and Through Loss. As mentioned above, the Drop Loss, AddLoss and Through Loss are preferably measured or calibrated at, forexample, the factory and are known (e.g. stored or received) bycontroller 70.

Controller 70 preferably operates so as to match the added channel powerlevel with the optical signal power level output from the ADM module 30.One way in which to accomplish this inventive goal is for the controller70 to control the amplification or gain imparted by add amp 50 to thechannels to be added to the WDM signal in the ADM module 30.Conceptually, the add channel power may be calculated by determining thedropped channel power as if the dropped channels had propagated throughthe ADM module 30. In other words, the add channel power is based on thedropped channel power plus an offset where the offset accounts for thegains and losses associated with the dropped channels actually passingthrough the ADM module 30. An exemplary equation to achieve this goal isas follows:P _(add) =P _(drop)+Offset  Equation (1)where

-   Offset=Drop Loss+Add Loss−Through Loss (respective losses associated    with a signal travelling the drop path, add path and through path of    ADM module 30 in dBm)-   P_(add)=the power per channel added (absolute power of added    channels divided by number of added channels, then converted to dBm)-   P_(drop) is the power per channel dropped (absolute power of dropped    channels as measured via coupler 42 divided by number of dropped    channels, then converted to dBm) substituting Offset in Equation (1)    leaves:    P _(add) =P _(drop)+(Drop Loss+Add Loss−Through Loss)  Equation (2)

The above equations are on a per-channel basis. Since ADM 30 may addmore than one channel, an additional calculation may be necessary inorder for controller 70 to command the appropriate output power level(add path amplification value) of the add amp 50. Specifically:P_(addtotal)=P_(add)+10Log N _(add)  Equation (3)where

-   P_(addtotal)=desired amplification in dBm at output of the add amp    50.-   N_(add)=number of added channels.    Substituting for P_(add) leaves:    P_(addtotal)=P_(drop)+(Drop Loss+Add Loss−Through Loss)+10Log N    _(add)  Equation (4)

In some cases such as when there is a fiber cut such that no channelswill be dropped while one or more channels are added by ADM 30 or whenADM 30 is otherwise asymmetric and adds but does not drop channels.Alternatively, the ADM module 30 may be replaced by an add module 32 asshown in FIG. 5 which only has the capability to add but not dropchannels. For any of the system configurations 10-14 shown in FIGS. 1-5,the add power calculation changes when no channels are being dropped.Specifically,P_(add)=P_(in)+OffsetNoDrop  Equation (5)where

-   P_(in)=the power per channel input to ADM 30 (absolute power of    input channels as measured via coupler 22 divided by number of input    channels, then converted to dBm)-   OffsetNoDrop=Add Loss−Through Loss    Substituting OffsetNoDrop in Equation (5) leaves:    P_(add)=P_(in)+(Add Loss−Through Loss)  Equation (6)    Again, this equation is on a per-channel basis. Therefore,    accounting for the total number of added channels leaves the    following equation for the add path amplification value:    P_(addtotal)=P_(add)+10LogN_(add)  Equation (7)    where-   P_(addtotal)=desired amplification in dBm at output of the add amp    50.-   N_(add)=number of added channels.    Substituting for P_(add) leaves:    P_(addtotal)=P_(in)+(Add Loss−Through Loss)+10Log N _(add)  Equation    (8)

Because the OADM systems 10-13 of FIGS. 1-4 and ADM system 14 of FIG. 5may process WDM signals comprised of more than one channel (wavelength),the controller 70 preferably has knowledge of the number of channelsinput, dropped and added. To that end, controller 70 may include aninput 75 which inputs information relating to the number of channelsbeing dropped and added with respect to ADM module 30. It is to beunderstood that a subset of this information may be sufficient for theinvention to operate correctly. For example, if the ADM module 30 issymmetric and always drops and adds a fixed number of channels, then itwould be sufficient to know these number(s). A controller 70 workingwith an asymmetric ADM module 30 generally requires both add and dropchannel count information.

The input 75 may be from a variety of sources one of which is a servicechannel which may be a separate wavelength of the WDM signal separatefrom the data wavelengths. Alternatively, the channel count informationmay be transmitted to the input 75 via a network separate from the WDMsignal such as an Ethernet connection, wireless connection or WWW (worldwide web) connection. If the number of channels being transmitted,dropped and added are fixed, the input 75 may simply be from a registeror other memory device operatively connected to the controller 70. Stillfurther, the channel count information may be manually input via, forexample, a craft terminal connected to input 75.

Input 75 may also be used to input the calibrated Drop Loss, ThroughLoss and Add Loss values. Alternatively, these values may be programmedinto or otherwise stored by controller 75 without using input 75.

The P_(addtotal) value calculated above may used to control the addamplifier 50. More specifically, the P_(addtotal) value may used bycontroller 70 to command the pump lasers (not shown) within add amp 50to tune the output power to P_(addtotal).

To increase the accuracy of the output power, a feedback loop ispreferably used. To that end, an add amp 50 output power measurement istaken via coupler 52. This add amp 50 output power measurement iscompared against P_(addtotal) to feedback control the add amp 50. Thisinvention contemplates using conventional feedback control methods inthis new environment to achieve more precise control.

In addition to controlling the added channel power level, it is alsopreferable to match the gain profiles of these two signals. One way inwhich to accomplish this inventive goal is for the gain profile of addamp 50 to substantially match the gain profile of the input amp 20 orotherwise substantially match the gain profile of the WDM signal inputto the ADM module 30. If the input amp 20 is used, a simple way toaccomplish this goal is to use the same type of amplifier for input amp20 and add amp 50. Since the added channels typically comprise only aportion of the total WDM spectrum the gain profile match need only bewith respect to the added channel spectrum and not necessarily the totalWDM spectrum.

The output of the ADM module 30 is preferably optically coupled to anoutput amp 60 as further shown in FIG. 1. The output amp 60 ispreferably a gain-flattened amplifier having a frequency response thatsubstantially flattens the frequency spectrum of the signal output fromamp 60. Such gain-flattened amplifiers are generally available andtypically include a plurality of amplifier stages, variable opticalattenuators, gain flattening filters and the like to impart therelatively flat gain profile. The inventive processing of the addedchannels which seeks to match the gain and frequency profile of theadded channels to the channels passing through the ADM module worksparticularly well with a gain-flattened output amp 60. The reason isthat the amp 60 need not compensate for a different power level orfrequency profile for the added channels and can thereby provide aoverall flatter gain response for the WDM signal.

The invention is not limited to the equations or examples describedabove in relation to FIG. 1. A variety of changes to the OADM system 10structure and the methods performed thereby are within the scope ofinvention.

Indeed, FIG. 2 shows one such variation on this theme which is to addone or more dispersion compensating elements to the OADM system 11. FIG.2 is the preferred embodiment in which a first dispersion compensatingelement (DCE) 90 is optically coupled to the input of the ADM module 30.Dispersion compensating element 90 provides dispersion compensation forthe optical signals output from the first amp stage 20 and input to theADM module 30 , and can include a variety of elements such as a segmentof known dispersion compensating fiber (DCF) or a dispersioncompensating Bragg grating or both.

An optional DCE 95 may also be coupled between the ADM module 30 and thedrop amp 40 as also shown in FIG. 2. It is noted that a dispersioncompensating element may also be provided at any appropriate locationwithin any one of the embodiments of the present invention.

Other differences between the embodiments of FIGS. 1 and 2 are the inputand drop path power values. By placing additional elements, namely DCEs90 and 95, in the input and drop paths the amount of signal attenuationon each path will increase. Such attenuations should be accounted forwhen calculating the appropriate power level output by add amp 50. Forexample, the DCE 90 loss should be subtracted from the input power levelP_(in). The DCE 95 loss should be accounted for by the power measurementtaken via coupler 42 since this power measurement point follows the DCE95.

FIG. 3 illustrates an alternative to achieving a desired add channelpower. Instead of controlling the add amp 50 output power as describedabove, a variable optical attenuator (VOA) 56 may be utilized to controlthe add channel power. For example, the add amp 50 may be set to arelatively high amplification value that ensures sufficientamplification for the add channels. This high amplification may then betuned by the VOA 56 to provide the desired add channel power value. Thecontroller 70 may exercise this control over VOA 56 in accordance withthe descriptions above. The VOA 56 may be a separate element or providedwithin the add amp 50.

Optional coupler 52 may be used to create a feedback loop to ensure thatthe add channel power is equal to the desired value.

In addition, both the add amp 50 and VOA 56 may be controlled bycontroller 70 to produce the desired add channel power value. In otherwords, the controller 70 may seek to find an appropriate balance betweenthe amplification imparted by add amp 50 and the attenuation imparted byVOA 56 to produce the desired add channel power value. In thisalternative, optional couplers 50, 52 may be used by controller tofeedback control the add amp 50 and the VOA 56.

Although using a VOA 56 is within the scope of invention, it isgenerally not preferred because of the additional expense andcomplexity. Moreover, the necessary control over the add channel powervalue may be appropriately performed by the add amp 50.

FIG. 1 illustrates a typical interface between the ADM module 30 and thedrop/add, output/input signal pathways. Namely, a demultiplexer 80 istypically used to separate the dropped WDM signals into componentsignals which may be supplied to a receiver (Rx). A multiplexer 85 isanother component of a typical interface in which a plurality of signalsfrom, for example, a transmitter (Tx) may be multiplexed into a WDMsignal and supplied to the ADM module 30. Such demultiplexers 80 andmultiplexers 85 are conventional elements the details of which are notnecessary for the understanding of the current invention.

FIG. 4 illustrates one of a variety of alternative interfaces. Insteadof multiplexer 85 and demultiplexer 80 as in FIG. 1, the added anddropped channels may be supplied from/to another span or amplifier (e.g.the span to which add amp is connected or the amplifier 100 to which thedrop amp 40 is connected). The illustrated amplifier 100 and span aremerely examples of interfaces and may include a variety of othercomponents. For example, the dropped channels may be supplied to anotherspan or fiber plant for transmission. If there is more than one droppedchannel, the signal being supplied will be a WDM-type signal.

FIG. 4 also illustrates a generalization of the add amp 50 and add amp50/VOA 56 illustrated in previous figures. This generalized version islabeled a gain element 58 and may include an add amp 50 as in FIGS. 1-2,a combination of add amp 50 and VOA 56 as in FIG. 3 or any othercombination of amplifiers and attenuators capable of adjusting the addpath amplification in response to a command from controller 70. The gainelement 58 may also be feedback controlled by controller 70 using thepower measurement taken via coupler 52.

Moreover, the drop amplifier 40 is not necessary for the invention tooperate. In other words, the focus of the present invention is on theadd path amplification and the drop path amplification is a useful,albeit not strictly necessary, addition to the invention.

FIG. 6 is an example of the method performed by the invention. Thismethod may be performed by controller 70 to control the add amp 50, theVOA 56 or gain element 58.

As shown in FIG. 6, the inventive method includes monitoring the ADM 30input power and the drop channel power. This may be accomplished asdescribed above and involves measuring power via couplers 22 and 42.

As reflected in the above equations, the ADM input power does not needto be monitored if the ADM module 30 drops at least one channel.Furthermore, the dropped channel power does not need to be monitored ifthe ADM module 30 does not drop any channels.

The method also inputs the drop, add and input channel counts. Asdiscussed above, only a subset of this information may be necessary suchas just the add and drop channel counts.

The method may then calculate the desired add path amplification value.This calculation may be performed as described above.

An optional step may then be performed to determine whether anysignificant changes to the monitored or input values have occurred. Thedegree of change sufficient to trigger a calculation and addamplification update may be adjusted or eliminated entirely to tune thesystem performance. The degree of change may be detected by tracking orotherwise storing previously monitored or input values. In addition, thechange may relate to only one or a subset of the all values monitored orinput. For example, at least a 2% change in the dropped channel powermay be deemed a “significant” change meriting controlling the add pathamplification to reach an updated value.

If a change has occurred (or when this step is eliminated), then themethod changes the add path amplification by controlling the add amp 50and/or the VOA 56. The method may continue in this fashion or may betriggered upon the occurrence of an event (e.g. the “any changes”decision step, the installation of systems 10-13; a time-out; or manualoperation may be used as a triggering event for execution of themethod).

FIGS. 7-11 further illustrate the concepts of the invention. FIG. 7 is atypical gain profile of a signal input to the ADM module 30. This gainprofile may be preexisting (e.g. in case input amp is not used) orimparted by the input amp 20 if part of the system. The gain profileshown in FIG. 7 is but one, non-limiting example of a typical gainprofile of a WDM signal.

FIG. 8 illustrates a typical gain profile of the WDM signal passingthrough ADM module 30 after dropping one or more channels onto the droppath. As shown therein, a gap in the profile exists and corresponds tothe channel(s) that were dropped.

FIG. 9 illustrates the channels to be added before amplification. Forexample, the added channel(s) output from the multiplexer 85 may have again profile as shown in FIG. 9. For simplicity of illustration, thegain profile is flat but that is not always the case.

FIG. 10 illustrates the result of the inventive processing which is tomatch the power level and gain profile of the WDM signal. Specifically,the channels to be added (FIG. 9) are amplified by amp 50 to a leveldetermined by controller 70.

If the preferred add amp 50 is used having a gain profile thatsubstantially matches the gain profile of the WDM signal input to theADM module 30, then the added channel(s) output from add amp 50 willpreferably look like those shown in FIG. 10. The preferred result isthat the combined signals (WDM signal after dropping and addingchannels) will appear as shown in FIG. 7. In other words, the ADM 20will appear power-transparent and insensitive to changes in the numberof channels input, dropped, or added and or changes in the respectivepower values of the input, dropped, and added channels.

For comparison sake, if the gain profiles do not match (e.g. the add amp50 has a flat profile), then the combined signals (WDM signal afterdropping and adding channels) may appear as shown in FIG. 11.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded asdeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. An optical communications apparatus for power balancing a wavelengthdivision multiplexed (WDM) signal output from an add/drop module addingat least one channel to and dropping at least one channel from a signalinput thereto, comprising: a gain element optically coupled to theadd/drop module and to an add channel port receiving the at least onechannel to be added; said gain element imparting optical gain to the atleast one channel received from the add channel port; said gain elementhaving a gain profile substantially matching a gain profile of thesignal input to the add/drop module; a controller operatively coupled tosaid gain element, said controller receiving a dropped channel powermeasurement of the at least one dropped channel dropped from theadd/drop module; said controller determining an add path amplificationvalue based on the dropped channel power measurement, a through lossassociated with a signal passing through the add/drop module, a droploss associated with a signal travelling a drop path of the add/dropmodule; and an add loss associated with a signal travelling an add pathof the add/drop module; and said controller controlling said gainelement according to the add path amplification value.
 2. The opticalcommunications apparatus for power balancing a wavelength divisionmultiplexed (WDM) signal according to claim 1, said controller receivinga number of channels to be added by the add/drop module; said controllerdetermining the add path amplification value based on the number ofchannels to be added, the dropped channel power measurement, the throughloss associated with a signal passing through the add/drop module, thedrop loss associated with a signal travelling a drop path of theadd/drop module; and the add loss associated with a signal travelling anadd path of the add/drop module; and said controller controlling saidgain element according to the add path amplification value.
 3. Theoptical communications apparatus for power balancing a wavelengthdivision multiplexed (WDM) signal according to claim 1, said controllerreceiving a number of channels to be dropped by the add/drop module;said controller determining the add path amplification value based onthe number of channels to be dropped, the dropped channel powermeasurement, the through loss associated with a signal passing throughthe add/drop module, the drop loss associated with a signal travelling adrop path of the add/drop module; and the add loss associated with asignal travelling an add path of the add/drop module; and saidcontroller controlling said gain element according to the add pathamplification value.
 4. The optical communications apparatus for powerbalancing a wavelength division multiplexed (WDM) signal according toclaim 1, said controller receiving a number of channels to be added anda number of channels to be dropped by the add/drop module; saidcontroller determining the add path amplification value based on thenumber of channels to be added, the number of channels to be dropped,the dropped channel power measurement, the through loss associated witha signal passing through the add/drop module, the drop loss associatedwith a signal travelling a drop path of the add/drop module; and the addloss associated with a signal travelling an add path of the add/dropmodule; and said controller controlling said gain element according tothe add path amplification value.
 5. The optical communicationsapparatus for power balancing a wavelength division multiplexed (WDM)signal according to claim 2, said controller determining the add pathamplification based on the following equation:P_(addtotal)=P_(drop)+(Drop Loss+Add Loss−Through Loss)+10Log N _(add)where P_(addtotal)=add path amplified power level in dBm, P_(drop)=perchannel power level of dropped signal output from drop port of add/dropmodule in dBm, Through Loss=loss associated with a signal passingthrough the add/drop module in dBm, Drop Loss=loss associated with asignal travelling a drop path of the add/drop module in dBm, AddLoss=loss associated with a signal travelling an add path of theadd/drop module in dBm, and N_(add)=number of added channels.
 6. Theoptical communications apparatus for power balancing a wavelengthdivision multiplexed (WDM) signal according to claim 1, furthercomprising: a coupler optically coupled to a drop output of the add/dropmodule, an optical-to-electrical converter optically coupled to saidcoupler, said optical to-electrical coupler receiving a portion of lightfrom the at least one dropped channel; said controller determining thedropped channel power measurement from an output of saidoptical-to-electrical converter.
 7. The optical communications apparatusfor power balancing a wavelength division multiplexed (WDM) signalaccording to claim 1, said controller receiving an added channel powermeasurement of the least one added channel being added to the inputsignal by the add/drop module; said controller feedback controlling saidgain element based on the added channel power measurement and the addpath amplification value.
 8. The optical communications apparatus forpower balancing a wavelength division multiplexed (WDM) signal accordingto claim 1, further comprising: an input amplifier optically coupled aninput port of the add/drop module and receiving a plurality of inputchannels; said gain element having a gain profile substantially matchinga gain profile of said input amplifier.
 9. The optical communicationsapparatus for power balancing a wavelength division multiplexed (WDM)signal according to claim 1, further comprising: a drop amplifieroptically coupled to the drop port of the add/drop module, said dropamplifier amplifying the dropped channels.
 10. The opticalcommunications apparatus for power balancing a wavelength divisionmultiplexed (WDM) signal according to claim 1, further comprising: anoutput amplifier optically coupled to the add/drop module; said outputamplifier amplifying the output of the add/drop module.
 11. The opticalcommunications apparatus for power balancing a wavelength divisionmultiplexed (WDM) signal according to claim 10, said output amplifierperforming gain flattening amplification for the signal output from theadd/drop module.
 12. The optical communications apparatus for powerbalancing a wavelength division multiplexed (WDM) signal according toclaim 1, wherein said gain element includes an add amplifier, saidcontroller controlling said add amplifier according to the add pathamplification value.
 13. The optical communications apparatus for powerbalancing a wavelength division multiplexed (WDM) signal according toclaim 1, wherein said gain element includes an add amplifier and avariable optical attenuator, said controller controlling said variableoptical attenuator according to the add path amplification value. 14.The optical communications apparatus for power balancing a wavelengthdivision multiplexed (WDM) signal according to claim 1, wherein saidgain element includes an add amplifier and a variable opticalattenuator, said controller controlling said variable optical attenuatorand said add amplifier according to the add path amplification value.15. A method of power balancing a wavelength division multiplexed (WDM)signal output from an add/drop module adding at least one channel to anddropping at least one channel from a signal input thereto, comprising:receiving a dropped channel power measurement of the least one droppedchannel dropped from the add/drop module; determining an add pathamplification value based on the dropped channel power measurement, athrough loss associated with a signal passing through the add/dropmodule, a drop loss associated with a signal travelling a drop path ofthe add/drop module; and an add loss associated with a signal travellingan add path of the add/drop module; controlling an add pathamplification of the add path according to the add path amplificationvalue; and substantially matching a gain profile of the add path with again profile of a signal input to the add/drop module.
 16. The methodaccording to claim 15, further comprising: receiving a number ofchannels to be added by the add/drop module; and determining the addpath amplification value based on the number of channels to be added,the dropped channel power measurement, the through loss associated witha signal passing through the add/drop module, the drop loss associatedwith a signal travelling a drop path of the add/drop module; and the addloss associated with a signal travelling an add path of the add/dropmodule.
 17. The method according to claim 15, further comprising:receiving a number of channels to be dropped by the add/drop module; anddetermining the add path amplification value based on the number ofchannels to be dropped, the dropped channel power measurement, thethrough loss associated with a signal passing through the add/dropmodule, the drop loss associated with a signal travelling a drop path ofthe add/drop module; and the add loss associated with a signaltravelling an add path of the add/drop module.
 18. The method accordingto claim 15, further comprising: receiving a number of channels to beadded and a number of channels to be dropped by the add/drop module; anddetermining the add path amplification value based on the number ofchannels to be added, the number of channels to be dropped, the droppedchannel power measurement, the through loss associated with a signalpassing through the add/drop module, the drop loss associated with asignal travelling a drop path of the add/drop module; and the add lossassociated with a signal travelling an add path of the add/drop module.19. The method according to claim 16, further comprising: determiningthe add path amplification based on the following equation:P_(addtotal)=P_(drop)+(Drop Loss+Add Loss−Through Loss)+10Log N _(add)where P_(addtotal)=add path amplified power level in dBm, P_(drop)=perchannel power level of dropped signal output from drop port of ADMmodule in dBm, Through Loss=loss associated with a signal passingthrough the add/drop module in dBm, Drop Loss=loss associated with asignal travelling a drop path of the add/drop module in dBm, AddLoss=loss associated with a signal travelling an add path of theadd/drop module in dBm, and N_(add)=number of added channels.
 20. Themethod according to claim 15, further comprising: preamplifying a signalinput to the add/drop module; said preamplification step imparting again profile substantially similar to a gain profile of the add path.21. The method according to claim 15, wherein the add path includes anadd amplifier; said controlling step controlling a gain of the addamplifier.
 22. The method according to claim 15, wherein the add pathincludes an add amplifier and a variable optical attenuator opticallycoupled thereto; said controlling step controlling a gain of the addamplifier and/or an attenuation of the variable optical attenuator. 23.The method according to claim 15, amplifying the output of the add/dropmodule with gain flattening amplification.